Large Ozone Depletion Research Articles

GUV long-term measurements of total ozone column and effective cloud transmittance at three Norwegian sites

Abstract. Measurements of total ozone column and effective cloud transmittance have been performed since 1995 at the three Norwegian sites Oslo/Kjeller, Andøya/Tromsø, and in Ny-Ålesund (Svalbard). These sites are a subset of nine stations included in the Norwegian UV monitoring network, which uses ground-based ultraviolet (GUV) multi-filter instruments and is operated by the Norwegian Radiation and Nuclear Safety Authority (DSA) and the Norwegian Institute for Air Research (NILU). The network includes unique data sets of high-time-resolution measurements that can be used for a broad range of atmospheric and biological exposure studies. Comparison of the 25-year records of GUV (global sky) total ozone measurements with Brewer direct sun (DS) measurements shows that the GUV instruments provide valuable supplements to the more standardized ground-based instruments. The GUV instruments can fill in missing data and extend the measuring season at sites with reduced staff and/or characterized by harsh environmental conditions, such as Ny-Ålesund. Also, a harmonized GUV can easily be moved to more remote/unmanned locations and provide independent total ozone column data sets. The GUV instrument in Ny-Ålesund captured well the exceptionally large Arctic ozone depletion in March/April 2020, whereas the GUV instrument in Oslo recorded a mini ozone hole in December 2019 with total ozone values below 200 DU. For all the three Norwegian stations there is a slight increase in total ozone from 1995 until today. Measurements of GUV effective cloud transmittance in Ny-Ålesund indicate that there has been a significant change in albedo during the past 25 years, most likely resulting from increased temperatures and Arctic ice melt in the area surrounding Svalbard.

Record-Breaking Increases in Arctic Solar Ultraviolet Radiation Caused by Exceptionally Large Ozone Depletion in 2020.

Measurements of solar ultraviolet radiation (UVR) performed between January and June 2020 at 10 Arctic and subarctic locations are compared with historical observations. Differences between 2020 and prior years are also assessed with total ozone column and UVR data from satellites. Erythemal (sunburning) UVR is quantified with the UV Index (UVI) derived from these measurements. UVI data show unprecedently large anomalies, occurring mostly between early March and mid‐April 2020. For several days, UVIs observed in 2020 exceeded measurements of previous years by up to 140%. Historical means were surpassed by more than six standard deviations at several locations in the Arctic. In northern Canada, the average UVI for March was about 75% larger than usual. UVIs in April 2020 were elevated on average by about 25% at all sites. However, absolute anomalies remained below 3.0 UVI units because the enhancements occurred during times when the solar elevation was still low.

Reactive nitrogen (NO<sub><i>y</i></sub>) and ozone responses to energetic electron precipitation during Southern Hemisphere winter

Abstract. Energetic particle precipitation (EPP) affects the chemistry of the polar middle atmosphere by producing reactive nitrogen (NOy) and hydrogen (HOx) species, which then catalytically destroy ozone. Recently, there have been major advances in constraining these particle impacts through a parametrization of NOy based on high-quality observations. Here we investigate the effects of low (auroral) and middle (radiation belt) energy range electrons, separately and in combination, on reactive nitrogen and hydrogen species as well as on ozone during Southern Hemisphere winters from 2002 to 2010 using the SOCOL3-MPIOM chemistry-climate model. Our results show that, in the absence of solar proton events, low-energy electrons produce the majority of NOy in the polar mesosphere and stratosphere. In the polar vortex, NOy subsides and affects ozone at lower altitudes, down to 10 hPa. Comparing a year with high electron precipitation with a quiescent period, we found large ozone depletion in the mesosphere; as the anomaly propagates downward, 15 % less ozone is found in the stratosphere during winter, which is confirmed by satellite observations. Only with both low- and middle-energy electrons does our model reproduce the observed stratospheric ozone anomaly.

Recent Arctic ozone depletion: Is there an impact of climate change?

After the well-reported record loss of Arctic stratospheric ozone of up to 38% in the winter 2010–2011, further large depletion of 27% occurred in the winter 2015–2016. Record low winter polar vortex temperatures, below the threshold for ice polar stratospheric cloud (PSC) formation, persisted for one month in January 2016. This is the first observation of such an event and resulted in unprecedented dehydration/denitrification of the polar vortex. Although chemistry–climate models (CCMs) generally predict further cooling of the lower stratosphere with the increasing atmospheric concentrations of greenhouse gases (GHGs), significant differences are found between model results indicating relatively large uncertainties in the predictions. The link between stratospheric temperature and ozone loss is well understood and the observed relationship is well captured by chemical transport models (CTMs). However, the strong dynamical variability in the Arctic means that large ozone depletion events like those of 2010–2011 and 2015–2016 may still occur until the concentrations of ozone-depleting substances return to their 1960 values. It is thus likely that the stratospheric ozone recovery, currently anticipated for the mid-2030s, might be significantly delayed. Most important in order to predict the future evolution of Arctic ozone and to reduce the uncertainty of the timing for its recovery is to ensure continuation of high-quality ground-based and satellite ozone observations with special focus on monitoring the annual ozone loss during the Arctic winter.

Quantifying the ozone and ultraviolet benefits already achieved by the Montreal Protocol.

Chlorine- and bromine-containing ozone-depleting substances (ODSs) are controlled by the 1987 Montreal Protocol. In consequence, atmospheric equivalent chlorine peaked in 1993 and has been declining slowly since then. Consistent with this, models project a gradual increase in stratospheric ozone with the Antarctic ozone hole expected to disappear by ∼2050. However, we show that by 2013 the Montreal Protocol had already achieved significant benefits for the ozone layer. Using a 3D atmospheric chemistry transport model, we demonstrate that much larger ozone depletion than observed has been avoided by the protocol, with beneficial impacts on surface ultraviolet. A deep Arctic ozone hole, with column values <120 DU, would have occurred given meteorological conditions in 2011. The Antarctic ozone hole would have grown in size by 40% by 2013, with enhanced loss at subpolar latitudes. The decline over northern hemisphere middle latitudes would have continued, more than doubling to ∼15% by 2013.

Missing driver in the Sun-Earth connection from energetic electron precipitation impacts mesospheric ozone.

Energetic electron precipitation (EEP) from the Earth’s outer radiation belt continuously affects the chemical composition of the polar mesosphere. EEP can contribute to catalytic ozone loss in the mesosphere through ionization and enhanced production of odd hydrogen. However, the long-term mesospheric ozone variability caused by EEP has not been quantified or confirmed to date. Here we show, using observations from three different satellite instruments, that EEP events strongly affect ozone at 60–80 km, leading to extremely large (up to 90%) short-term ozone depletion. This impact is comparable to that of large, but much less frequent, solar proton events. On solar cycle timescales, we find that EEP causes ozone variations of up to 34% at 70–80 km. With such a magnitude, it is reasonable to suspect that EEP could be an important part of solar influence on the atmosphere and climate system.

What would have happened to the ozone layer if chlorofluorocarbons (CFCs) had not been regulated?

Abstract. Ozone depletion by chlorofluorocarbons (CFCs) was first proposed by Molina and Rowland in their 1974 Nature paper. Since that time, the scientific connection between ozone losses and CFCs and other ozone depleting substances (ODSs) has been firmly established with laboratory measurements, atmospheric observations, and modeling studies. This science research led to the implementation of international agreements that largely stopped the production of ODSs. In this study we use a fully-coupled radiation-chemical-dynamical model to simulate a future world where ODSs were never regulated and ODS production grew at an annual rate of 3%. In this "world avoided" simulation, 17% of the globally-averaged column ozone is destroyed by 2020, and 67% is destroyed by 2065 in comparison to 1980. Large ozone depletions in the polar region become year-round rather than just seasonal as is currently observed in the Antarctic ozone hole. Very large temperature decreases are observed in response to circulation changes and decreased shortwave radiation absorption by ozone. Ozone levels in the tropical lower stratosphere remain constant until about 2053 and then collapse to near zero by 2058 as a result of heterogeneous chemical processes (as currently observed in the Antarctic ozone hole). The tropical cooling that triggers the ozone collapse is caused by an increase of the tropical upwelling. In response to ozone changes, ultraviolet radiation increases, more than doubling the erythemal radiation in the northern summer midlatitudes by 2060.

Estimation of Arctic ozone loss in winter 2004/05 based on assimilation of EOS MLS and SBUV/2 observations

AbstractIn this paper we present a new technique for the estimation of ozone loss in the stratospheric polar vortex based on the assimilation of Earth Observing System Microwave Limb Sounder (EOS MLS) and Solar Backscatter Ultraviolet radiometer (SBUV/2) ozone observations in the Met Office data assimilation system. We focus on the northern winter of 2004/05, which was exceptionally cold in the Arctic stratosphere, with associated large ozone depletion due to heterogeneous chemistry. Our ozone loss estimate, which was calculated for the 1 February to 10 March 2005 period, peaks at 450 K (approximately 17–18 km), and is 0.6 ppmv at that isentropic level (our loss estimate for the vortex core only was somewhat higher (1.0 ppmv) and indicates uncertainties related to mixing at the vortex edge). This value is similar to or smaller than results from other studies, which estimate ozone loss in this period to be in the 0.6–1.2 ppmv range. When combined with results from other studies that estimate ozone loss occurring outside our assimilation period, we obtain an estimate of 0.8–1.2 ppmv for ozone loss from early January to early March 2005.We find a second maximum in ozone loss for the 1 February–10 March period near 650 K (approximately 25 km) of around 0.4 ppmv. This is a lower figure than found in other studies, but ozone loss is actually much stronger at this level outside the vortex in a low‐ozone pocket in the Aleutian anticyclone, likely due to the NOx catalytic cycle. Our results show that the ozone data assimilation method we have used to estimate ozone loss is very promising, and can lead to potentially more accurate ozone‐loss estimates than other methods. © Royal Meteorological Society and Crown Copyright, 2008.

The continuing environmental threat of nuclear weapons: Integrated policy responses

Humans have come to the realization that pollution of the atmosphere with gases and particles in the past 50 years is the dominant cause of atmospheric change [Intergovernmental Panel on Climate Change, 2007]. While land‐use change can produce large regional effects, ozone depletion, global warming, and nuclear smoke all are humandriven problems that have actual or potential global adverse impacts on our fragile environment, each with severe consequences for humanity. These effects were, or would be, inadvertent and unplanned consequences of normal daily activities, the defense policies of many nations, and nuclear proliferation. Thus, we must seek ways of continuing our normal lives while protecting ourselves from environmental catastrophe.

Ozonesonde observations in the Arctic during 1989–2003: Ozone variability and trends in the lower stratosphere and free troposphere

We have studied the interannual and longer‐term variations in ozone profiles over the Arctic from 1989 to 2003 using ozonesonde observations from seven northern high‐latitude stations. The ozonesonde data have been carefully examined and made as internally consistent as possible. The homogenized measurements are analyzed with a statistical model. In both the stratosphere and troposphere the largest long‐term changes have occurred in the late winter/spring period (January–April), the period of greatest interannual variability. In the stratosphere, layer ozone amounts correlate highly with proxies for the stratospheric circulation (100 hPa eddy heat flux averaged over 45–70°N), polar ozone depletion (the calculated volume of polar stratospheric clouds combined with the effective equivalent stratospheric chlorine) and tropopause height. At altitudes between 50 and 70 hPa, we estimate that chemical polar ozone depletion accounted for up to 50% of the March ozone variability. Negative trends in the lower stratosphere prior to 1997 can be attributed to the combined effect of dynamical changes, the impact of aerosols from the Mt. Pinatubo eruption and winters of relatively large chemical ozone depletion. Since 1996–1997 the observed increase in lower stratospheric ozone can be attributed primarily to dynamical changes. In the free troposphere, a statistically significant increase of 11.3 ± 1.8% over 15 years is observed which also maximizes in the January to April period (16.0 ± 3.1% over 15 years). The model suggests that this can be attributed to the effects of changes in the Arctic Oscillation.

The influence of the several very large solar proton events in years 2000–2003 on the neutral middle atmosphere

Solar proton events (SPEs) are known to have caused changes in constituents in the Earth’s polar neutral middle atmosphere. The past four years, 2000–2003, have been replete with SPEs. Huge fluxes of high energy protons entered the Earth’s atmosphere in periods lasting 2–3 days in July and November 2000, September and November 2001 and October 2003. The highly energetic protons produce ionizations, excitations, dissociations and dissociative ionizations of the background constituents, which lead to the production of HO x (H, OH, HO 2) and NO y (N, NO, NO 2, NO 3, N 2O 5, HNO 3, HO 2NO 2, ClONO 2, BrONO 2). The HO x increases lead to short-lived ozone decreases in the polar mesosphere and upper stratosphere due to the short lifetimes of the HO x constituents. Large mesospheric ozone depletions (>70%) due to the HO x enhancements were observed and modeled as a result of the very large July 2000 SPE. The NO y increases lead to long-lived stratospheric ozone changes because of the long lifetime of the NO y family in this region. Polar total ozone depletions >1% were simulated in both hemispheres for extended periods of time (several months) as a result of the NO y enhancements due to the very large SPEs.

Ozone trends: A review

Ozone plays a very important role in our atmosphere because it protects any living organisms at the Earth's surface against the harmful solar UVB and UVC radiation. In the stratosphere, ozone plays a critical role in the energy budget because it absorbs both solar UV and terrestrial IR radiation. Further, ozone in the tropopause acts as a strong greenhouse gas, and increasing ozone trends at these altitudes contribute to climate change. This review contains a short description of the various techniques that provided atmospheric ozone measurements valuable for long‐term trend analysis. The anthropogenic emissions of substances that deplete ozone (chlorine‐ and bromine‐containing volatile gases) have increased from the 1950s until the second half of the 1980s. The most severe consequence of the anthropogenic release of ozone‐depleting substances is the “Antarctic ozone hole.” Long‐term observations indicate that stratospheric ozone depletion in the southern winter‐spring season over Antarctica started in the late 1970s, leading to a strong decrease in October total ozone means. Present values are only approximately half of those observed prior to 1970. In the Arctic, large ozone depletion was observed in winter and spring in some recent years. Satellite and ground‐based measurements show no significant trends in the tropics but significant long‐term decreasing trends in the northern and southern midlatitudes (of the order of 2–4% per decade in the period from 1970 to 1996 and an acceleration in trends in the 1980s). Ozone at northern midlatitudes decreased by −7.4±2% per decade at 40 km above mean sea level, while ozone loss was small at 30 km. Large trends were found in the lower stratosphere, −5.1±1.8% at 20 km and −7.3±4.6% at 15 km, where the bulk of the ozone resides. The possibility of a reduction in the observed trends has been discussed recently, but it is very hard to distinguish this from the natural variability. As a consequence of the Montreal Protocol process, the emissions of ozone‐depleting substances have decreased since the late 1980s. Chlorine is no longer increasing in the stratosphere, although the total bromine amount is still increasing. Considering anthropogenic emissions of substances that deplete ozone, the turnaround in stratospheric ozone trends is expected to take place in the coming years. However, anthropogenic climate change could have a large influence on the future evolution of the Earth's ozone shield.

Northern midlatitude stratospheric ozone dilution in spring modeled with simulated mixing

Measurements have shown a substantial decrease in Northern midlatitude ozone, which has only partially been explained by chemical models. The large ozone depletions determined for the Arctic vortex in recent winters will ultimately spread out and dilute the midlatitudes and thus contribute to the observed decrease. Here we have followed the ozone‐depleted air inside the Arctic vortex in 1995 and 1997 during April and May with high‐resolution reverse domain‐filling (RDF) trajectory calculations. The resulting average midlatitude (30°–60°N) stratospheric ozone dilution in May is 2.9% and 2.6% of the 1979 column ozone in 1995 and 1997, respectively, or about 40% of the observed depletion. Nearly realistic mixing was introduced by a regridding procedure between successive 7‐day long RDF calculations. Low‐resolution grid point models give too much mixing, causing an overestimate of the calculated dilution. A recovery of about 12% of the midlatitude dilution in May 1997 is calculated with a photochemical box model, but is not included in the number given above.

Long-term changes of the upper stratosphere as seen by Japanese rocketsondes at Ryori (39°N, 141°E)

Abstract. Wind and temperature profiles measured routinely by rockets at Ryori (Japan) since 1970 are analysed to quantify interannual changes that occur in the upper stratosphere. The analysis involved using a least square fitting of the data with a multiparametric adaptative model composed of a linear combination of some functions that represent the main expected climate forcing responses of the stratosphere. These functions are seasonal cycles, solar activity changes, stratospheric optical depth induced by volcanic aerosols, equatorial wind oscillations and a possible linear trend. Step functions are also included in the analyses to take into account instrumental changes. Results reveal a small change for wind data series above 45 km when new corrections were introduced to take into account instrumental changes. However, no significant change of the mean is noted for temperature even after sondes were improved. While wind series reveal no significant trends, a significant cooling of 2.0 to 2.5 K/decade is observed in the mid upper stratosphere using this analysis method. This cooling is more than double the cooling predicted by models by a factor of more than two. In winter, it may be noted that the amplitude of the atmospheric response is enhanced. This is probably caused by the larger ozone depletion and/or by some dynamical feedback effects. In winter, cooling tends to be smaller around 40-45 km (in fact a warming trend is observed in December) as already observed in other data sets and simulated by models. Although the winter response to volcanic aerosols is in good agreement with numerical simulations, the solar signature is of the opposite sign to that expected. This is not understood, but it has already been observed with other data sets.Key words. Atmospheric composition and structure (evolution of one atmosphere; pressure · density · and temperature) · Meteorology and atmospheric dynamics (middle atmosphere dynamics)

Evidence for a substantial role for dilution in northern mid‐latitude ozone depletion

Satellite measurements have shown a significant decline in ozone at mid‐latitudes, and this has only partially been explained by chemical models. Here we show that the large ozone depletion in the Arctic vortex in spring 1997 leads to a dilution of northern mid‐latitude ozone after break‐up of the vortex of about 3% or more than 1/3 of the observed change in TOMS total ozone at 30–60°N from May 1979 to May 1997. The dilution in spring 1993–96 is crudely estimated to be 2–4%. These results indicate that dilution plays a substantial role in the mid‐latitude ozone depletion in spring and summer, when the harmful effects of increases in UV radiation are largest.

Ozone depletion in and below the Arctic Vortex for 1997

The winter 1996/97 was quite unusual with late vortex formation and polar stratospheric cloud (PSC) development and subsequent record low temperatures in March. Ozone depletion in the Arctic vortex is determined using ozonesondes. The diabatic cooling is calculated with PV‐theta mapped ozone mixing ratios and the large ozone depletions, especially at the center of the vortex where most PSC existence was predicted, enhances the diabatic cooling by up to 80%. The average vortex chemical ozone depletion from January 6 to April 6 is 33, 46, 46, 43, 35, 33, 32 and 21 % in air masses ending at 375, 400, 425, 450, 475, 500, 525, and 550 K (about 14–22 km). This depletion is corrected for transport of ozone across the vortex edge calculated with reverse domain‐filling trajectories. 375 K is in fact below the vortex, but the calculation method is applicable at this level with small changes. The column integrated chemical ozone depletion amounts to about 92 DU (21%), which is comparable to the depletions observed during the previous four winters.

Effects of interannual variation of temperature on heterogeneous reactions and stratospheric ozone

A two‐dimensional chemical/dynamical/microphysical model is used to calculate the effect of temperature interannual variability on stratospheric ozone. Two effects associated with temperature variations are considered in this paper: First, the effect on the rate coefficient of heterogeneous reactions occurring at the surface of sulfate aerosols, in particular for the enhanced sulfate aerosols following the eruption of Mount Pinatubo, and second, the effect of temperature interannual variations on the formation of polar stratospheric clouds (PSCs) and the spring Antarctic ozone depletion. The interannual variabilities of ozone concentration affected by both the processes are evaluated. The model results show that during winter 1992–1993, stratospheric temperature at high latitudes in the northern hemisphere is colder than during winter 1991–1992, leading to a larger rate of heterogeneous conversion from inactive chlorine to active chlorine in sulfate aerosols during winter 1992–1993 than during winter 1991–1992 at northern high latitudes. As a result, ozone reduction due to heterogeneous reactions following the eruption of Mount Pinatubo is enhanced in a colder winter (1992–1993) and reduced in a warmer winter (1991–1992). This result indicates that the enhanced heterogeneous conversion associated with colder temperature can explain, in part, the large ozone depletion observed by the total ozone mapping spectrometer (TOMS) experiment during winter 1992–1993. The model results also show that during winters 1986 to 1990, the formation of PSCs over Antarctica exhibits a strong interannual variation, due to temperature variability. In the colder years (1987 and 1989) the surface area of PSCs (a major factor to determine the rate of heterogeneous conversion from inactive chlorine to active chlorine) increases, and ozone concentration decreases. In the warmer years (1986 and 1988) the results are opposite. The calculated ozone interannual variation is consistent with the satellite observation (TOMS), indicating that the interannual variation of the formation of PSCs likely plays an important role for the interannual variability of the spring Antarctic ozone hole.

Aerosol‐induced chemical perturbations of stratospheric ozone: Three‐dimensional simulations and analysis of mechanisms

An atmospheric general circulation model is coupled with a stratospheric photochemical model to simulate the chemical/dynamical perturbations associated with background and volcanically perturbed aerosols in the lower stratosphere. The present work focuses on short‐term anomalies at middle and high latitudes in the northern hemisphere, where large ozone depletions have been observed in late winter and early spring, particularly following the eruption of Mount Pinatubo. Five fully coupled simulations are analyzed, corresponding to a control case with only gas phase chemistry, and cases including heterogeneous chemistry on background aerosols, on El Chichón‐type, and on Pinatubo‐type aerosols. It is found that heterogeneous reactions occurring on sulfate aerosols (background or postvolcanic) can strongly perturb the chemical partitioning in the lower stratosphere, leading to significant ozone depletion through enhanced chlorine, bromine, and odd‐hydrogen catalytic cycles. In the Arctic lower stratosphere, the maximum zonal and March monthly mean local ozone reductions (with respect to the control case) can exceed 15% for the background aerosol case, 40% for the El Chichón case, and 50% for the Pinatubo case. The corresponding zonal mean total column ozone decreases are roughly 5% and 15% for the background and volcanic aerosol cases, respectively. In the most extreme case tested (post‐Pinatubo), a large ozone depletion below 30 mbar is offset to some extent by an ozone increase above that level. The results of a sensitivity study (in which the aerosols are distributed closer to the tropics, as might occur early after an eruption at low latitude) lead to relatively small total ozone depletions at northern high latitudes, and small ozone increases in the tropical lower stratosphere. The reduced impact on total ozone at high latitudes is associated both with local ozone increases above 30 mbar and with poleward transport of enhanced ozone from the tropical lower stratosphere. The ozone increase at low latitudes is the net result of compensating changes in the catalytic destruction cycles involving odd‐nitrogen and chlorine species activated by heterogeneous processes at the low temperatures and abundant sunlight found near the tropical tropopause. Our simulations indicate that ozone variations triggered by volcanic injections of aerosols depend on the global distribution as well as the abundance of the particles and their evolution over time, and on the initial dynamical‐radiative‐chemical state of the atmosphere, which itself exhibits large seasonal and interannual variability.

Evidence of large scale ozone depletion within the Arctic Polar Vortex 94/95 based on airborne LIDAR measurements

Measurements with an airborne lidar performed during the Arctic winter 1994/95 show, that the ozone mixing ratio declined by about 50% throughout the polar vortex in the period from the beginning of February to mid of March in a sharp defined altitude band from 420 K to 520 K potential temperature. The simultaneous detection of the stratospheric aerosol density provides evidence that this depletion was not caused by dynamical processes but by chemical reactions.

Spring polar ozone behavior

It has been recognized since the commencement of Antarctic ozone measurements during the IGY that spring southern polar total ozone amount is less than spring northern polar total ozone amount. More importantly, since 1980 there has been a decline in the minimum spring total ozone value, from 250 DU in 1980 to 125 DU in 1987 and below 120 in 1991. This decline occurs within the winter polar vortex, which acts as a containment vessel preventing polar ozone from escaping to lower latitudes and excluding ozone-rich air from the polar region. Ozone decrease can be explained in terms of heterogeneous reactions of chlorine and nitrogen reservoir molecules on polar stratospheric clouds. These clouds form in the lower polar stratosphere during winter when temperatures in the Antarctic are sufficiently low to create water ice clouds. Clouds involving nitric acid form at higher temperatures. Chlorine reservoirs such as HCl are converted to Cl 2, which is photodissociated in the presence of sunlight. The resulting Cl reacts with O 3 to form ClO. Measurements of ClO and other species give agreement of theory and experiment within the uncertainties of the measurement. Heterogeneous chemistry accounts for most of the ozone hole. A small amount of ozone loss is also observed above the polar stratospheric cloud level, implying another mechanism, either chemical or dynamical. Above 25 km, formation of ozone-destroying odd nitrogen in the upper stratosphere by energetic electrons and the existence of any trend is still an open question. There is much less ozone depletion in the Arctic. This is the result of a less stable polar vortex and warmer temperatures, which reduce polar stratospheric cloud formation. There is strong evidence that tropospheric forcing within or just outside the vortex leads to adiabatic cooling with resulting cloud formation. During such events ozone-poor tropospheric air is transported into the stratosphere. In the Arctic this can result in the transport of long-lived hydrocarbons. Subsequent reactions lead to the formation of HCl, reducing the effect of Cl. There is also production of HO 2, which accelerates ozone loss due to chlorine. There are also small areas of large and rapid ozone depletion termed miniholes. Ozone-poor air from these regions can propagate to lower latitudes, as can the air from within the vortex, when it disintegrates in late spring. Data from the BUV ozone-measuring instrument on the Nimbus 4 satellite indicate the existence of October 1970 Antarctic ozone of only 250 DU. This is evidence of the existence of ozone loss with only CH 3Cl and low concentrations of CFCs as chlorine sources.